Traditionally, many commercial applications of high temperature materials have been filled by Ni-, Co-, and Fe-based metal alloys. These materials function well at temperatures below about 800.degree. C., but rapidly lose strength upon exposure to higher temperatures. Thus, in the field of high temperature materials, researchers have focused on the use of heat resistant fibers to reinforce both metallic and ceramic materials. These high strength composites possess a unique combination of high strength, temperature stability, and low density. This allows for their use in materials for aerospace, automotive, and industrial applications.
Silicon-containing materials are known reinforcements for composite materials. These composites potentially possess high toughness levels and good performance characteristics, thereby making them highly suitable for applications which require light-weight structural materials having high elasticity, high strength, shapability, heat stability, electrical conductivity and heat conductivity. These composites are being increasingly investigated for structural applications.
It is known that many fiber-matrix combinations undergo extensive chemical reaction or interdiffusion between the fiber and matrix materials, each of which is likely chosen for the contribution of specific mechanical and/or physical properties to the resulting composite. Such reaction or interdiffusion can lead to serious degradation in strength, toughness, ductility, temperature stability and oxidation resistance. Some changes may result from the difference in the thermal expansion coefficients of the materials.
SiC reinforcements have a relatively low thermal expansion coefficient (3-4 ppm/.degree.C.) compared to the metallic alloys and compounds (12-20 ppm/.degree.C.) that they are being developed to reinforce. In general, reinforcements that have high elastic moduli have low thermal expansion coefficients. The thermal expansion mismatch between the matrix material and the SiC reinforcement leads to stress at the reinforcement/matrix interface, and subsequent failure due to fracture of either the reinforcement or the matrix material.
To address this issue, the idea of a compliant layer which is less stiff than the matrix has been proposed to accommodate the stress due to thermal expansion coefficient differences. In a recent review, Petrasek (NASA Conference Publication 10039, pages 8-1 to 8-13) describes these issues for intermetallic matrix composites. One serious problem with the compliant layer concept is related to the chemical compatibility of the compliant layer, which is normally a metal alloy or compound, with both the SiC reinforcement and the matrix material alloy.
To compensate for this problem and others, a variety of coatings have been suggested for reinforcements intended for use in fiber-matrix composites. For example, U.S. Pat. No. 4,340,636 discloses a surface treatment for the formation of a carbon-rich coating on a stoichiometric SiC substrate filament. Similarly, U.S. Pat. No. 4,315,968 discloses coating SiC filaments with a thin coating of Si-rich SiC. U.S. Pat. No. 3,811,920 discusses applying a thin layer of TiC to a filamentary substrate having a SiC surface layer. Boron nitride has also been tried as a SiC coating, as in U.S. Pat. No. 4,642,271.
Intermetallic matrix materials have experienced problems similar to those enumerated hereinabove when combined with reinforcements to produce high performance composites. The problems being experienced in this technology field are generally a result of the fact that the matrix material technology and fiber technology have evolved independent of one another, resulting in chemical and mechanical incompatability of the precursor materials used to produce composites of the fiber-matrix-type. The foregoing citations demonstrate various attempts within the field to overcome the inherent shortcomings of these composites by using coating materials to provide the needed characteristics or compatibility.
However, composite materials which have employed techniques and coatings such as the foregoing nonetheless remain limited for high temperature application by concerns regarding the thermomechanical stability, thermochemical stability, oxidation resistance and high temperature fatigue resistance encountered in atmospheric conditions at temperatures above 800.degree. C. A specific problem encountered with a number of these coatings relates to the chemical reactivity of the coating with the matrix materials, which manifests itself in the failure of the mechanical and physical performance of the material in high temperature environments. This is particularly true in the case of metallic coatings on SiC reinforcements since extensive chemical reactions are known to occur at SiC/metal interfaces.
Accordingly, an object of the subject invention is to provide a multi-layer coating for SiC reinforcements which permits the use of the reinforcement in composite materials for use at high temperatures.
Another object of the invention is to provide a multi-layer coating for SiC reinforcements which prevents chemical reaction between the fiber and the matrix material.
A further object is to provide a composite which contains coated SiC reinforcements which maintain high strength and toughness and resist oxidation at high temperatures.